The present invention relates to heat transfer elements of the type found in rotary regenerative heat exchangers.
Rotary regenerative heat exchangers are commonly used to transfer heat from flue gases exiting a furnace to the incoming combustion air. Conventional rotary regenerative heat exchangers, such as that shown as 1 in FIG. 1, have a rotor 12 mounted in a housing 14. The housing 14 defines a flue gas inlet duct 20 and a flue gas outlet duct 22 for the flow of heated flue gases 36 through the heat exchanger 1. The housing 14 further defines an air inlet duct 24 and an air outlet duct 26 for the flow of combustion air 38 through the heat exchanger 1. The rotor 12 has radial partitions 16 or diaphragms defining compartments 17 therebetween for supporting baskets (frames) 40 of heat transfer elements. The rotary regenerative heat exchanger 1 is divided into an air sector and a flue gas sector by sector plates 28, which extend across the housing 14 adjacent the upper and lower faces of the rotor 12.
FIG. 2 depicts an end elevation view of an example of an element basket 40 including a few elements 10 stacked therein. While only a few elements 10 are shown, it will be appreciated that the basket 40 will typically be filled with elements 10. As can be seen in FIG. 2, the elements 10 are closely stacked in spaced relationship within the element basket 40 to form passageways 70 between the elements 10 for the flow of air or flue gas.
Referring to FIGS. 1 and 2, the hot flue gas stream 36 is directed through the gas sector of the heat exchanger 1 and transfers heat to the elements 10 on the continuously rotating rotor 12. The elements 10 are then rotated about axis 18 to the air sector of the heat exchanger 1, where the combustion air stream 38 is directed over the elements 10 and is thereby heated. In other forms of rotary regenerative heat exchangers, the elements 10 are stationary and the air and gas inlet and outlet portions of the housing 14 rotate.
FIG. 3 depicts portions of conventional elements 10 in stacked relationship, and FIG. 4 depicts a cross-section of one of the conventional elements 10. Typically, elements 10 are steel sheets that have been shaped to include one or more various notches 50 and undulations 65.
Notches 50, which extend outwardly from the element 10 at generally equally spaced intervals, maintain spacing between adjacent elements 10 when the elements 10 are stacked as shown in FIG. 3, and thus form sides of the passageways 70 for the air or flue gas between the elements 10. Typically, the notches 50 extend at a predetermined angle (e.g. 90 degrees) relative to the fluid flow through the rotor (12 of FIG. 1).
In addition to the notches 50, the element 10 is typically corrugated to provide a series of undulations (corrugations) 65 extending between adjacent notches 50 at an acute angle Au to the flow of heat exchange fluid, indicated by the arrow marked “A” in FIG. 3. The undulations 65 have a height of Hu and act to increase turbulence in the air or flue gas flowing through the passageways 70 and thereby disrupt the thermal boundary layer that would otherwise exist in that part of the fluid medium (either air or flue gas) adjacent to the surface of the element 10. The existence of an undisrupted fluid boundary layer tends to impede heat transfer between the fluid and the element 10. The undulations 65 on adjacent elements 10 extend obliquely to the line of flow. In this manner, the undulations 65 improve heat transfer between the element 10 and the fluid medium. Furthermore, the elements 10 may include flat portions (not shown), which are parallel to and in full contact with the notches 50 of adjacent elements 10. For examples of other heat transfer elements 10, reference is made to U.S. Pat. Nos. 2,596,642; 2,940,736; 4,396,058; 4,744,410; 4,553,458; and 5,836,379.
Although such elements exhibit favorable heat transfer rates, the results can vary rather widely depending upon the specific design and the dimensional relationship between the notches and the undulations. For example, while the undulations provide an enhanced degree of heat transfer, they also increase the pressure drop across the heat exchanger (1 of FIG. 1). Ideally, the undulations on the elements will induce a relatively high degree of turbulent flow in that part of the fluid medium adjacent to the elements, while the notches will be sized so that the fluid medium that is not adjacent to the elements (i.e., the fluid near the center of the passageways) will experience a lesser degree of turbulence, and therefore much less resistance to flow. However, attaining the optimum level of turbulence from the undulations can be difficult to achieve since both the heat transfer and the pressure loss tend to be proportional to the degree of turbulence that is produced by the undulations. An undulation design that raises the heat transfer tends to also raise the pressure loss and, conversely, a shape that lowers the pressure loss tends to lower the heat transfer as well.
Design of the elements must also present a surface configuration that is readily cleanable. To clean the elements, it has been customary to provide soot blowers that deliver a blast of high-pressure air or steam through the passages between the stacked elements to dislodge any particulate deposits from the surface thereof and carry them away leaving a relatively clean surface. To accommodate soot blowing, it is advantageous for the elements to be shaped such that when stacked in a basket the passageways are sufficiently open to provide a line of sight between the elements, which allows the soot blower jet to penetrate between the sheets for cleaning. Some elements do not provide for such an open channel, and although they have good heat transfer and pressure drop characteristics, they are not very well cleaned by conventional soot blowers. Such open channels also allow for the operation of a sensor for measuring the quantity of infrared radiation leaving the element. Infrared radiation sensors can be used to detect the presence of a “hot spot”, which is generally recognized as a precursor to a fire in the basket (40 of FIG. 2). Such sensors, commonly known as “hot spot” detectors, are useful in preventing the onset and growth of fires. Elements that do not have an open channel prevent infrared radiation from leaving the element and from being detected by the hot spot detector.
Thus, there is a need for a rotary regenerative heat exchanger heat transfer element that provides decreased pressure loss for a given amount of heat transfer and that is readily cleanable by a soot blower and compatible with a hot spot detector.